Abstract

In the Science Buddies project Human-Powered Energy, you can learn about the basics of magnetic induction, or how moving magnets can be used to generate an electric current. The detailed physics of how a changing magnetic field can induce an electric current are described by Faraday's law and Lenz's law (see the Bibliography for more information about these two laws). For a more advanced version of the Human-Powered Energy project idea, try setting up an experiment to prove or demonstrate these two laws, using the Shaking Up Some Energy Kit from our partner Home Science Tools. Since this is an abbreviated project idea, Science Buddies will not give you an exact procedure to follow. Instead, you will need to come up with your own hypothesis and procedure to test it. The suggestions below can help you get started.

• Examine the equation form of Faraday's law, which states that the voltage generated in a coil of wire is proportional to the number of turns in the coil, and the rate of change of the magnetic flux. How can you manipulate either one of these two variables and measure the resulting change in voltage? Here are a few hints:
  • The rate of change of magnetic flux depends on how fast the magnets are moving through the tube. Rather than shaking the generator (which would make the magnets' velocity difficult to measure), construct a longer tube and drop the magnets through it from a known height, which should allow you to calculate their velocity when they pass through the coil.
  • In order to properly measure the voltage, you will need to use a multimeter set to measure AC voltage, or an oscilloscope. If you do not have access to either tool, you can use LEDs in series as a very rough measurement of the peak voltage (as described in the original Human-Powered Energy project idea). Keep in mind that voltages add in series, and the forward voltage drop of a red LED is roughly 2.2 V.
• Lenz's law describes how the polarity of the voltage induced in a coil depends on the direction a magnet's poles are facing and the direction in which the magnet is moving. Connect two LEDs to your coil with opposite polarities, meaning the LEDs should be wired in parallel, but facing opposite directions—the anode (long leg) of one LED should be connected to the cathode (short leg) of the other LED, and vice versa. How can you use this configuration to demonstrate Lenz's law? When you shake the generator, will the LEDs flash at the same time, or will they alternate? Here are a few hints:
  • The timing of the LED flashes may be difficult to see with the naked eye. Many modern smartphones and digital cameras have a "slow motion" video capture option, which will record video at 60 or even 120 frames per second. Use this ability to film the LEDs as you shake the generator. Use a media player that lets you step through a video frame-by-frame to closely examine the timing of the LED flashes.
  • Construct the tube for your wire coil using a transparency sheet (available at Amazon.com or an office supply store) instead of cardstock. This will allow you to see the magnet moving inside the coil. Rather than shaking the generator with your hand, which would be difficult to film, set up a longer tube and drop the magnet through it from a known height. Set up your camera so you can film the magnet falling through the tube and see the LEDs flashing in the same frame, as shown in Figure 1. This will allow you to correlate the position and velocity of the magnet within the tube with the timing and brightness of the LED flashes. Do your findings match up with your predictions based on Lenz's law?

Materials and Equipment

In addition to the kit from our partner Home Science Tools, you will need some other materials to complete this project. Here are some suggestions based on Figure 1 from the Summary:

• Transparency sheets, available from Amazon.com
• Corrugated cardboard
• Scissors
• Craft knife or utility knife
• Super glue
• Tape
• Smartphone or digital camera with slow-motion video capture mode

Careers

If you like this project, you might enjoy exploring these related careers:

Contact Us

If you have purchased a kit for this project from Science Buddies, we are pleased to answer your questions.

In your email, please follow these instructions:

1. What is your Science Buddies kit order number?
2. Please describe how you need help as thoroughly as possible:
   
   Examples
   
   Good Question I'm trying to do Experimental Procedure step #5, "Scrape the insulation from the wire. . ." How do I know when I've scraped enough?
   Good Question I'm at Experimental Procedure step #7, "Move the magnet back and forth . . ." and the LED is not lighting up.
   Bad Question I don't understand the instructions. Help!
   Good Question I am purchasing my materials. Can I substitute a 1N34 diode for the 1N25 diode called for in the material list?
   Bad Question Can I use a different part?

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